1. Field of the Invention
The invention relates to a printing unit for a rotary offset printing press having a plate cylinder bearing a printing plate suitable for offset-printing operation free of dampening solution, and an inking unit with a multiplicity of inking rollers, is provided for inking the printing plate during operation with a printing ink suitable for offset-printing operation free of dampening solution.
In such a printing unit, it is incumbent upon the plate cylinder to transfer, by means of the inking unit, printing ink which has been applied to the plate cylinder to an offset cylinder cooperating therewith and provided, in turn, for inking up stock or printing material with ink that which been accepted from the plate cylinder. In the process of inking up the printing ink, it has been found to be disadvantageous that particles with low adhesion to the printing material, such as paper fibers, for example, in cases wherein paper is used as the printing material, are transferred to the offset cylinder and to the printing plate to which they may become attached.
In contrast with an offset printing unit which employs dampening solution and in which, in particular, a difference in circumferential speeds of the plate cylinder, on the one hand, and a dampening roller contacting the plate cylinder, on the other hand, is provided, the danger arises, when performing offset printing free of dampening solution in a printing unit of the aforementioned type, that lint particles may accumulate on the printing plate, with the consequent formation of a correspondingly defective printed image on the printing material or stock.
An offset printing unit employing dampening solution has become known heretofore, for example, from the published German Patent Document DE 18 08 909 C3, wherein, in order to remove impurities from the plate cylinder, at least one of the applicator rollers of the dampening unit or the inking unit has a circumferential speed that is different from that of the plate cylinder. The resultant mode of operation, however, is considered unsuitable for use in a printing unit of the aforementioned type. The basis therefor lies in the nature of a printing plate suitable for offset printing free of dampening solution. Such a printing plate, namely, has a non-metallic surface coating with a far lower resistance to wear than that of a printing plate which has been formed for offset printing using dampening solution.